Core: Fix unicode handling in HTTPClient

fixes apache#7821

Without the fix, tests would fail with
```
expected: struct<1: id: required int (unique ID 🤪), 2: data: required string>
 but was: struct<1: id: required int (unique ID ?), 2: data: required string>
```

core/src/main/java/org/apache/iceberg/rest/HTTPClient.java

@@ -24,6 +24,7 @@
 import java.io.UncheckedIOException;
 import java.net.URI;
 import java.net.URISyntaxException;
+import java.nio.charset.StandardCharsets;
 import java.util.Map;
 import java.util.function.Consumer;
 import java.util.stream.Collectors;
@@ -471,13 +472,13 @@ public HTTPClient build() {
 
   private StringEntity toJson(Object requestBody) {
     try {
-      return new StringEntity(mapper.writeValueAsString(requestBody));
+      return new StringEntity(mapper.writeValueAsString(requestBody), StandardCharsets.UTF_8);
     } catch (JsonProcessingException e) {
       throw new RESTException(e, "Failed to write request body: %s", requestBody);
     }
   }
 
   private StringEntity toFormEncoding(Map<?, ?> formData) {
-    return new StringEntity(RESTUtil.encodeFormData(formData));
+    return new StringEntity(RESTUtil.encodeFormData(formData), StandardCharsets.UTF_8);
   }
 }

core/src/test/java/org/apache/iceberg/catalog/CatalogTests.java

@@ -73,19 +73,19 @@ public abstract class CatalogTests<C extends Catalog & SupportsNamespaces> {
   // Schema passed to create tables
   protected static final Schema SCHEMA =
       new Schema(
-          required(3, "id", Types.IntegerType.get(), "unique ID"),
+          required(3, "id", Types.IntegerType.get(), "unique ID 🤪"),
           required(4, "data", Types.StringType.get()));
 
   // This is the actual schema for the table, with column IDs reassigned
   private static final Schema TABLE_SCHEMA =
       new Schema(
-          required(1, "id", Types.IntegerType.get(), "unique ID"),
+          required(1, "id", Types.IntegerType.get(), "unique ID 🤪"),
           required(2, "data", Types.StringType.get()));
 
   // This is the actual schema for the table, with column IDs reassigned
   private static final Schema REPLACE_SCHEMA =
       new Schema(
-          required(2, "id", Types.IntegerType.get(), "unique ID"),
+          required(2, "id", Types.IntegerType.get(), "unique ID 🤪"),
           required(3, "data", Types.StringType.get()));
 
   // another schema that is not the same